1. Field of the Invention
The invention relates to sonar transducers and in particular to elliptical shell flextensional transducers.
2. Discussion of the Prior Art
Flextensional transducers are used to generate and radiate high power acoustic energy at low frequencies, typically in the range 200-800 Hz.
The construction of an elliptical shell transducer comprises a shell of an elliptical cylindrical form into which a piezo-electric stack or stacks is fitted along the major axis. These stacks consist of a number of piezo electric plates between which are sandwiched metal electrodes; these in turn being electrically connected in parallel. The ends of the shell are closed by end plates which are retained against the ends of the shell by tie bars.
When an alternating voltage is applied to the electrodes a vibration is generated along the major axis of the stack, this being transmitted into the shell, which due to its shape, increases the amplitude on the minor axis of the shell.
The normal method of assembling an elliptical shell flextensional transducer is by applying a load on the minor axis of the shell by means of a press of suitable size to cause an extension of the major axis such that the piezo-electric stack may be inserted, the final adjustments being made by the fitment of shims between the ends of the stack and the inner wall of the shell. This necessitates a relatively large working clearance to allow for fitting the shims.
When the load is removed from the minor axis, the major axis reduces in length and hence a stress is applied to the stack due to the action of the shell.
The major disadvantages of this type of assembly are:
1. the clearances required for assembly do not allow for the maximum advantage to be gained from the strain energy stored within the shell after loading; and PA1 2. there is difficulty in maintaining a uniform stress on the piezo electric stack without a very high standard of engineering and quality control, since very small differences in wall thickness of the shell causes asymmetic loading of the stack. PA1 a hollow cylindrical flexural shell, elliptical in cross section and open at both ends; PA1 at least one linear stack of piezo-electric elements fitted along the major axis of the ellipse between the opposed internal walls of the shell; PA1 two metal inserts located one at each end of the major axis between the shell wall and the corresponding end of the transducer stack and shaped in cross section to maintain the elliptical shape of the shell; and complementary wedge-shaped portions interposed between each insert and the corresponding stack end. PA1 (a) locating the shell on a supporting mandrel; PA1 (b) compression moulding a low shear modulus rubber coating, for example neoprene, over the outer surface of the shell to form a lip seal integral therewith on each end of the shell; PA1 (c) assembling end-plates to the shell and tightening tie-bars between the end plates so as to give the required compression of the end plate seals between each end plate and its respective shell end.
When designing an elliptical shell flextensional transducer it is essential to stress the piezo electric stack to a precise value, since when it is deployed into water the increasing hydrostatic pressure with depth progressively reduces the stress on the piezo-electric stack and hence a limit is reached beyond which the transducer cannot be driven without damage.
Flextensional transducers are normally sealed by means of end plates, however because they are capable of high power operation and thus the large amplitude flexing of the elliptical shell which occurs creates difficulties in water-tight sealing between the shell and end-plates since the sealing must be effective without limiting shell movement.
In order to operate there must be a pre-stress load applied by the elliptical shell to the transducer stacks. Operation over a wide range of pressure-depths requires that some form of pressure-balancing arrangements is provided.
Conventional pressure compensation or balancing systems have a number of operational disadvantages. The most common types of pressure balancing systems are air filled bladders and scuba type systems of which the latter use bottled compressed air coupled to a divers pressure balanced valve. The bladder method is severely limited as the volume of air in the cavity of the transducer is inversely proportional to the external hydrostatic pressure. The resulting reduction of the available swept volume for the active surface progressively lowers operating efficiency as the hydrostatic pressure is increased. The scuba system is a large and often relatively heavy appendage to a sonar transducer. In operation it can use large quantities of air if frequent changes in operating depth are required or if there are large unwanted depth excursions due to the effects of ocean swell on the deployment platform.
In a conventional design of flextensional transducer the dimensions of the shell are calculated to utilize the first and sometimes other flexural modes of vibration along the entire length of the oval cylinder. The shell has therefore a single resonance frequency and a finite bandwidth associated with each flexural mode.